add_submenu_page file callback

Description

Under the recommendation of the community I have decided to file a track regarding add_submenu_page. It currently allows for a callback function in which a dev could include a file but how about allowing add_submenu_page an argument to include a file in replacement of a callback function? This would remove several lines of code and clean up many developers functions.php files as well as custom admin page files.

I actually don't really like the idea of using a file here. Unfortunately, it makes it very easy for bad developers to expose a form to the point where direct file POSTs could essentially result in CSRF.